1. Field of the Invention:
This invention relates in general to identification-friend or foe systems.
2. Prior Art
The problem of identifying friends or foes with radar systems has existed for some time and interrogating stations have developed and transmitted code signals which when detected by friendly aircraft, for example, will respond by sending a coded response signal to the interrogating station. The codes are known to the interrogating and responding stations and make it difficult for intruders to falsely identify themselves. However, in such systems it is possible for a foe to monitor the interrogating and responding signals and to transmit a signal identical to a response previously monitored. Since such signal is identical to a responding signal that was proper at the time transmitted, the interrogating station might be confused or misled by the duplicate signal. It is also possible to jam the entire interrogation-answer system by frequently transmitting signals which merely repeat proper signals and codes since the interrogating station will be continually receiving spurious coded signals identical to those previously received. It is also possible for an enemy to locate apparatus answering an interrogation signal by transmitting a duplicate of an interrogating code which will be answered by responding stations and thus the enemy may locate the positions of the responding stations.
It is possible to eliminate or reduce the possibility of interference by continuously changing the interrogation and responding codes as a function of time. Such systems and apparatus are described for example in application Ser. No. 76,272. Such systems include interrogating and responding apparatus controlled by time standards and in a manner which is known of both stations such that the codes vary as a function by time and the responding signal will be transmitted only if the interrogating signal has the proper time relationship. Also, the responding signal varies as a function of time known to both the interrogating and the responding stations and based on a time standard similar to that utilized in the interrogating station.
However, due to the inaccuracy of time standards and because of differing transit times, which occur due to varying distances between interrogating and answering stations, it is possible for an enemy to detect the responding signal and re-transmit it so as to provide an interfering signal into the system.